The present invention relates to an electric discharge machine capable of curtailing a dead time during which an electrode moves backward immediately after a short-circuit occurs between the electrode and a workpiece to be machined during its operation, and then moves forward to start the machining operation again.
FIG. 1 is a schematic diagram showing the construction of a conventional electric discharge machine of this type. In FIG. 1, reference numeral 1 designates an electrode; 2, a workpiece to be machined; 3, a table; 4, a Y-axis motor for moving the table 3 in the direction of Y-axis; 5, an X-axis motor for moving the table 3 in the direction of X-axis; 6, a Z-axis motor for moving the electrode 1 in the direction of Z-axis; 7, a voltage detection unit for detecting an interelectrode gap voltage between the electrode 1 and the workpiece 2; 8, a shortcircuit detection unit for detecting a shortcircuit occurring between the electrode 1 and the workpiece 2 from the interelectrode gap voltage detected by the voltage detection unit 7; and 9, a movement instruction unit for calculating data to be applied to each motor 4, 5, or 6.
An operation of this conventional machine will be described with reference to FIG. 2 showing an example of movement of the electrode 1. During the machining, the electrode 1 moves spirally to machine the workpiece 2 along a machining locus 101 as instructed by the movement instruction unit 9. Upon an occurrence of a shortcircuit between the electrode 1 and the workpiece 2, the forward movement operation along the machining path 101 instructed by the movement instruction unit 9 is interrupted at a position 102 and the electrode 1 is then caused to move backward in a predetermined direction 103. Upon recovery from the shortcircuit, the electrode 1 is caused to return in reverse 104 to the position 102 at which its forward movement has been interrupted, so as to resume the machining operation along the machining path 105 instructed by the movement instruction unit 9.
Since the conventional electric discharge machine is constructed as described above, the backward movement of the electrode 1 is carried out according to one suitable mode selected by the operator from the group of predetermined backward movement modes, the thus selected mode being unchanged during the machining operation. Thus, with such a conventional machine, there have been some cases where the backward movement may result in redundancy depending on the machining path or the electrode configuration. More specifically, in the case where at least a part of the electrode 1 moves along the already machined path, there is a possibility that such a backward movement results in the fact that, no matter how the electrode 1 moves backward, it is impossible to recover from the shortcircuit between the electrode 1 and the workpiece 2 quickly resulting in requiring a long time to recover the shortcircuit.